Generally, the system of IEEE802.16e in progress of international standardization for the broadband wireless access system consists of a mobile station (MS), a base station (BS) and an ASA (authentication service authorization) as an authentication management server. In this case, a common physical layer (PHY) and medium access control (MAC) layer are defined between the MS and the BS.
FIG. 1 is diagram of an exemplary format of a scan request (e.g., MOB-SCN-REQ) message. FIG. 2 is diagram of an exemplary format of a scan response (e.g., MOB-SCN-RSP) message. An MS requests a scanning interval from a currently accessed serving BS to measure a signal quality of a neighboring BS and to decide a target BS that will be accessed in handover. The request is performed via a scan request (MOB-SCN-REQ) message illustrated in FIG. 1. In response to the request, the serving BS transmits a scan response (MOB-SCN-RSP) message, as illustrated in FIG. 2, including information associated with scan to the MS.
2. Sleep Mode
A broadband wireless access system supports a sleep mode to minimize power consumption of an MS. An MS in sleep mode acts according to a sleep interval that is increased by a fixed rate. The sleep interval consists of a listening window and a sleep window.
A value of the listening window is fixed via a sleep request/response message. For the listening window, the MS is instructed via a broadcast MAC management message (MOB-TRF-IND) transmitted from the BS to indicate whether there exists downlink traffic intended for the MS and whether ranging needs to be performed for uplink communication maintenance and appropriate downlink coding type maintenance according to signal quality. The sleep window is an interval during which the MS receives minimal downlink signals from the BS in order to save power. For the sleep window, the MS performs a scanning of neighboring BSs for handover, a ranging for uplink communication maintenance and appropriate downlink coding type maintenance according to signal quality.
FIG. 3 is diagram of an exemplary format of a sleep request (e.g., MOB-SLP-REQ) message. FIG. 4 is diagram of an exemplary format of a sleep response (e.g., MOB-SLP-RSP) message. Initialization of the sleep mode is performed by exchanging a sleep request (MOB-SLP_REQ) message and a sleep response (MOB-SLP-RSP) message between the MS and the BS. Specifically, to enter the sleep mode from a normal operation mode, the MS transmits a sleep request (MOB-SLP-REQ) message, as illustrated in FIG. 3, to the BS. The BS then transmits a sleep response (MOB-SLP-RSP) message, as illustrated in FIG. 4, to the MS.
3. Idle Mode
Idle mode enables an MS to save power consumption by receiving only a periodically transmitted downlink broadcast traffic message (MOB-PAG-ADV) when moving through an area controlled by several BSs without registering with a specific BS. In the idle mode, the MS has no signaling requirement for handover. By restricting an action associated with scanning, the MS may save resources, such as power and a connection ID.
FIG. 5 is diagram of an exemplary format of a de-registration request (e.g., DREG-REQ) message. Referring to FIG. 5, the MS transmits a de-registration request (DREG-REQ) message to the BS for idle mode initialization. In response to the message, the BS then transmits a de-registration command (DREG-CMD) message including information associated with idle mode admission of the MS. After transmitting the DREG-CMD message to the MS, the BS releases management resources such as a basic connection for MAC management message exchange with the MS, a primary connection, and a secondary connection for standard-based messages (DHCP, TFTP, etc.).
FIG. 6 is diagram of an exemplary format of a de-registration command (e.g., DREG-CMD) message. As such, FIG. 6 illustrates action codes included in the DREG-CMD message. An MS performs an action according to an instruction indicated by the action code. For instance, if a code is 0X05, an MS performs a de-registration procedure in a corresponding BS and initializes an idle mode. Moreover, even if the MS does not request the DREG-REQ message, the BS may transmit a DREG-CMD (unsolicited DREG-CMD) message having an action code 0X05 to cause the MS to enter the idle mode.
As mentioned in the above explanation, the MS transmits one of a scan request (MOB-SCN-REQ), a sleep request (MOB-SLP-REQ) and a de-registration request (DREG-REQ) message to the BS and then awaits one of a scan response (MOB-SCN-RSP), a sleep response (MOB-SLP-RSP) and a de-registration command (DREG-CMD) message from the BS in response.
FIG. 7 is a signal flow diagram illustrating an exemplary case where scan request and response messages are lost. FIG. 8 is a signal flow diagram illustrating an exemplary case where sleep request and response messages are lost. FIG. 9 is a signal flow diagram illustrating an exemplary case where de-registration request and command messages are lost. Referring to FIGS. 7-9, due to the limited radio link resources and wireless environmental factors, the MOB-SCN-REQ, MOB-SLP-REQ and DREG-REQ messages transmitted from the MS may not be delivered normally to the BS or the MOB-SCN-RSP, MOB-SLP-RSP and DREG-CMD messages transmitted from the BS in response may not be delivered normally to the MS.
In such case, since a processing procedure for a next action of the MS is not defined in a current broadband wireless access system, it is highly probable that the MS or BS may perform a wrong action.
In the idle mode, the BS releases the connections (basic, primary and secondary connections) and transmits the DREG-CMD message to the MS. Therefore, if the MS fails to receive the DREG-CMD message and retransmits the DREG-REQ message, there is no available connection (basic connection). Furthermore, even if the MS does not request idle mode initialization via the DREG-REQ message, if the DREG-CMD (unsolicited DREG-CMD) message transmitted to cause the MS to enter the idle mode is lost, the MS continues to operate in a normal operation mode. However, after deciding that the MS has entered the idle mode, the BS releases management resources for the corresponding MS. Thereafter, the MS is unable to transmit messages to the BS.